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Foreign spying was considered a major threat during World War II, and FBI director J. Edgar Hoover called on citizen sleuths to help provide surveillance. "Keep your eyes and ears open," he wrote in PM. "If you have grounds for suspicion, call the nearest Field Office."

Post-9/11, we're again being drafted to watch for suspicious activity, but surveillance has also gone on autopilot. Ever more sophisticated and pervasive technology is watching for trouble—and just watching—all the time. But the question remains: What does that mean for privacy? —Emily Masamitsu